1. Field of the Invention
Aspects of the present invention relate to a communication system. More particularly, aspects of the present invention relate to Cyclic Redundancy Check (CRC) encoding in a communication system.
2. Description of the Related Art
A Cyclic Redundancy Check (CRC) refers to an error detection method employed in a communication system for reliably detecting an error in communicated information by comparing a CRC code generated by a sender to a CRC code generated by a receiver. Examples of communication systems that implement CRC are systems based on a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard and a 3GPP LTE-Advanced (LTE-A) standard, which are hereafter referred to as an LTE system and an LTE-A system, respectively. While use of CRC is beneficial for all communicated information in the LTE system and the LTE-A system, CRC is particularly beneficial for the communication of control channel information.
An example of a control channel message in the LTE system and an LTE-A system is a Physical Downlink Control Channel (PDCCH), which carries scheduling assignments and other control information. In the LTE or LTE-A system, the PDCCH is transmitted from a base station, which is also referred to as an evolved Node B (eNB), and received by a User Equipment (UE). Hereafter, a UE that is an intended recipient of the PDCCH message is referred to as an intended UE and a UE that is not an intended recipient of the PDCCH message is referred to as an unintended UE.
The transmission by an eNB and the reception by a UE of PDCCH messages in the LTE or LTE-A system will be described below with reference to FIG. 1 and FIG. 2, respectively.
FIG. 1 illustrates a structure in an eNB for transmission of PDCCH messages in an LTE or LTE-A system according to the related art.
Referring to FIG. 1, the structure in the eNB for the transmission of PDCCH messages in the LTE or LTE-A system includes Cyclic Redundancy Check (CRC) encoders 102-A and 102-B, Tail-Biting Convolution Code (TBCC) encoders 104-A and 104-B, rate matchers 106-A and 106-B, a multiplexor/aggregator 108, a scrambler 110, a modulator 112, a Multiple-Input and Multiple-Output (MIMO) Transmission (TX) processor 114, a resource mapper 116, a multiplexor 118, and an Inverse Fast Fourier Transform (IFFT) unit 120.
In FIG. 1, the CRC encoder 102-A encodes a PDCCH message Msg_A for error detection. The PDCCH message Msg_A was generated by a PDCCH message generator (not shown). A CRC field of the CRC encoded message is scrambled by a UE identifier of an intended UE. The UE identifier may also be referred to as a Radio Network Temporary Identifier (RNTI) in the LTE or LTE-A system. There are different types of RNTIs in the LTE or LET-A system, e.g., a Cell-RNTI (C-RNTI), a Random Access-RNTI (RA-RNTI), etc. The purpose of scrambling the CRC field by the UE identifier is to allow only the intended UE (or UEs) to correctly detect the PDCCH message while having the PDCCH message appear as an erroneous message to other UEs that are unintended UEs.
The TBCC encoder 104-A encodes the CRC encoded message using a ⅓ rate TBCC. The rate matcher 106-A selects a number of coded bits from all the coded bits for the message generated by the TBCC encoder 104-A. The purpose of the rate matcher 106-A is to match the number of transmitted coded bits of the PDCCH message with an amount of resources allocated for transmission of that PDCCH message.
The CRC encoder 102-B, the TBCC encoder 104-B, the rate matcher 106-B perform the same operations on PDCCH message Msg_B as the operations described above by the CRC encoder 102-A, the TBCC encoder 104-A, and the rate matcher 106-A on the PDCCH message Msg_A, and thus a description thereof will be omitted for brevity.
Multiple PDCCH messages, such as Msg_A and Msg_B, can be transmitted in one Transmission Time Interval (TTI). The multiple PDCCH messages transmitted in one TTI can be intended for one or multiple UEs. In the LTE or LTE-A system, one TTI corresponds to one subframe, which has a time span of 1 ms. The coded bits from multiple PDCCH messages are multiplexed together by the multiplexor/aggregator 108, scrambled by the scrambler 110 using a cell-specific scrambling sequence, and then modulated by the modulator 112 using a Quadrature Phase Shift Keying (QPSK) modulation.
If multiple antennas are deployed, the MIMO TX processor 114 may perform MIMO processing on the modulation symbols output from the modulator 112. For example, in the LTE system, a Space-Frequency Block Coding (SFBC) or a SFBC-Frequency Switched Transmit Diversity (FSTD) scheme can be applied to PDCCH modulation symbols for an eNB with two transmit antennas or four transmit antennas, respectively.
After MIMO processing, a stream of modulation symbols are generated for each transmit antenna (or antenna port). Upon further interleaving, the modulation symbols are mapped by the resource mapper 116 to Resource Elements (REs) on a time-frequency grid of a subframe that comprises multiple Orthogonal Frequency-Division Multiplexing (OFDM) symbols. The multiplexor 118 multiplexes the signal output from the resource mapper 116 and provides the multiplexed signal to the IFFT unit 120. The IFFT unit 120 performs an IFFT operation on the signal output from the multiplexor 118 and outputs the transmission signal.
The eNB transmits multiple PDCCHs by multiplexing the multiple PDCCHs and mapping the modulation symbols for the multiple PDCCH messages to different time-frequency resources. In order to achieve satisfactory performance of the PDCCH, PDCCH messages can be transmitted using different message sizes and different amounts of resources to suit the needs of sending different PDCCH messages to UEs with different channel conditions. It would be cumbersome and inefficient if the transmission format for each PDCCH needed to be signaled to a corresponding intended UE (or UEs). Instead, in the LTE or LTE-A system, only the total amount of resources allocated for the PDCCH is signaled by a Physical Control Format Indicator Channel (PCFICH). As will be discussed below, the UEs employ blind decoding to detect PDCCH messages.
FIG. 2 illustrates a structure in a UE for reception of PDCCH messages in an LTE or LTE-A system according to the related art.
Referring to FIG. 2, the structure in the UE for the reception of PDCCH messages in the LTE or LTE-A system includes CRC decoders 202-1 to 202-K, TBCC decoders 204-1 to 204-K, rate de-matchers 206-1 to 206-K, a de-multiplexor/de-aggregator 208, a descrambler 210, a demodulator 212, a MIMO Reception (RX) processor 214, a resource de-mapper 216, a de-multiplexor and equalizer 218, and a Fast Fourier Transform (FFT) unit 220.
In FIG. 2, the FFT unit 220 performs an FFT operation on a received signal. The de-multiplexor and equalizer 218 de-multiplexes and equalizes the received signal output by the FFT unit 220. The resource de-mapper 216 de-maps symbols from REs on a time-frequency grid of a subframe that comprises multiple OFDM symbols. The MIMO RX processor 214 performs MIMO reception processing on the demapped symbols, which are then demodulated by the demodulator 212 and descrambled by the descrambler 210. The de-multiplexor/de-aggregator 208 de-multiplexes and de-aggregates the signal output from the demodulator 212 and provides the de-multiplexed and de-aggregated signals to the rate de-matchers 206-1 to 206-K. The rate de-matchers 206-1 to 206-K rate de-match the signal output from the de-multiplexor/de-aggregator 208 and each provide a rate de-matched signal to a corresponding one of the TBCC decoders 204-1 to 204-K. The TBCC decoders 204-1 to 204-K TBCC decode the signals provided from the rate de-matchers 206-1 to 206-K and each provide a TBCC decoded signal to a corresponding one of the CRC decoders 202-1 to 202-K. The CRC decoders 202-1 to 202-K CRC decode the signals provided from the TBCC decoders 204-1 to 204-K and output blind decoding hypotheses 1 to K. The process performed between the de-multiplexor/de-aggregator 208 and the CRC decoders 202-1 to 202-K constitutes the receiver blind decoding of the PDCCH messages.
There are a limited number of possibilities where a PDCCH can be transmitted, and a limited number of possible PDCCH message formats (i.e., Downlink Control Information (DCI) formats). In addition, to limit the total number of blind decodings a UE needs to perform, a number of possibilities where a PDCCH message can be transmitted to a specific UE, and the DCI formats a specific UE needs to detect, are further limited. A UE attempts decoding of a PDCCH message, assuming a possible DCI format on a possible resource location. If the UE is able to successfully decode the message, the CRC for the message passes CRC error detection. Moreover, if the PDCCH message is intended for the UE, the CRC scrambling sequence should match with the RNTI of the UE. A UE can attempt decoding of a PDCCH assuming all possible combinations of DCI formats and resource locations that are eligible for that UE. By doing so, the eNB can eliminate the extra signaling of the DCI formats and the location of the messages to corresponding UEs.
The multiplexing and mapping of PDCCH messages to time-frequency resources in an LT LTE or LTE-A E system will be described below with reference to FIG. 3.
FIG. 3 illustrates multiplexing and mapping of PDCCH messages to time-frequency resources in an LTE or LTE-A system according to the related art.
Referring to FIG. 3, upon encoding, rate matching, scrambling, modulation, and MIMO processing, modulation symbol quadruplets are formed. The modulation symbol quadruplets are interleaved and mapped to Resource Element Groups (REG). Note that every REG comprises four resource elements for data transmission. Accordingly, every REG can carry one modulation symbol quadruplet.
A PDCCH message is transmitted on an aggregation of one or several consecutive Control Channel Elements (CCEs), where a control channel element corresponds to 9 resource element groups. In other words, every CCE corresponds to 9 modulation symbol quadruplets, which in turn corresponds to 36 modulation symbols. For example, as shown FIG. 3, the first PDCCH is transmitted using 36 modulation symbols s0-s35, which is transmitted on 9 resource element groups. These 9 resource element groups form the first CCE. The second PDCCH message is transmitted using another 36 modulation symbols s36-s71, which is transmitted on another 9 resource element groups. These 9 resource element groups form the second CCE.
In the LTE or LTE-A system, the number of REGs not assigned to the PCFICH or a Physical Hybrid Automatic Repeat-reQuest (ARQ) Indicator Channel (PHICH) is NREG. The CCEs available in the LTE or LTE-A system are numbered from 0 and NCCE-1, where NCCE=└NREG/9┘. The PDCCH supports multiple formats, examples of which are shown below in Table 1. A PDCCH message consisting of n consecutive CCEs may only start on a CCE fulfilling i mod n=0, where i is the CCE number. Multiple PDCCH messages can be transmitted in a subframe.
TABLE 1PDCCHNumberNumberNumber offormatof CCEsof REGsPDCCH bits01972121814424362883872576
A PDCCH message, also referred to as Downlink Control Information (DCI), can be transmitted using 1, 2, 4, or 8 CCEs. As indicated above, a PDCCH message consisting of n consecutive CCEs may only start on a CCE fulfilling i mod n=0, where i is the CCE number. As a result, the CCE aggregation exhibits a tree structure, an example of which is shown in FIG. 4.
FIG. 4 illustrates a tree structure of CCE aggregation in an LTE or LTE-A system according to the related art.
The LTE system includes mechanisms to prevent PDCCH message detection error events. For example, scrambling the CRC of a PDCCH message with the RNTI of the intended UE (or UEs) allows only the intended UE (or UEs) to detect the PDCCH message and pass the CRC error detection successfully while other UEs will either not be able to detect the PDCCH message or not to be able to pass the CRC error detection. In addition, measures are taken to limit the number of blind decodings, which reduces the probability of CRC false detection (i.e., an error event that passes CRC error detection for an erroneous PDCCH detection). For example, a PDCCH consisting of n consecutive CCEs may only start on a CCE fulfilling i mod n=0, where i is the CCE number. In summary, the PDCCH is designed such that the intended UE (or UEs) can blindly detect the PDCCH message successfully with a high probability.
In the LTE-A system, carrier aggregation and cross-carrier scheduling are supported. When carrier aggregation is implemented in the LTE-A system, each carrier is referred to as a component carrier. In order for the PDCCH messages in one downlink component carrier to schedule downlink transmission in another downlink component carrier, or to schedule uplink transmission in an uplink component carrier that is paired with another downlink component carrier, the Carrier Indication Field (CIF) is needed in the PDCCH message for the purpose of cross-carrier scheduling.
In addition, the UE monitors a search space (among the total PDCCH control channel region) for the PDCCH messages intended for a specific component carrier. The search spaces for different component carriers are different. When an eNB transmits a PDCCH message to a UE, the eNB transmits the PDCCH message on one (or few) resources within the search space corresponding to the intended component carrier. If the resource used to transmit the PDCCH message is disposed in the portion of the search space for the component carrier that is non-overlapping with the search spaces of any other component carrier, the CIF field of the PDCCH message is redundant because the component carrier information is already implied based on the resources used to transmit the PDCCH message. This scenario can occur quite often. In other words, often times, the CIF is redundant. On the other hand, the PDCCH false detection probability has led to a concern regarding LTE or LTE-A system reliability and operational efficiency. Accordingly, it would be beneficial to improve the false detection capability of the PDCCH message, as well as other control channel messages and payload data.
Therefore, a need exists for techniques for improving the false detection capability of CRC encoded information.